Signing the Transaction but Not the Decision: Whisper Attacks and a Binding Defense for AP2
Research shows AP2 agent-payment signatures can be manipulated into valid but wrong carts; proposed A-VIP defense binds signed intent to purchases.
A study demonstrates Whisper attacks on the AP2 agent payment protocol, where ordinary product-description text steers shopping agents into carts that pass every cryptographic check but no longer match user intent. Using Gemini Flash-Lite models specified by AP2's default sample agents, three attacks succeeded at 90%, 56%, and 73.3%, with the vulnerability spanning seventeen Google models, three agent frameworks, cross-vendor anchors, and Google's consumer assistant. The proposed A-VIP defense treats signed intent as a capability grant, binding credential lookups to sessions and cart lines to seen listings, blocking the first two attacks with zero false positives while surfacing unauthorized spending. The authors release A-VIP code, machine-checked invariants, and AP2-WhisperBench with 1,544 evaluation scenarios.
- Attacks exploit the gap between signed transactions and agent decisions
- Attack success rates of 90%, 56%, and 73.3% on Gemini Flash-Lite defaults
- Vulnerability spans 17 Google models, three frameworks, and Google's consumer assistant
- A-VIP blocks two attacks with zero false positives, flags unauthorized spending
- Releases AP2-WhisperBench suite of 1,544 scenarios
Full article284 words · extracted from arxiv.org · click to collapse
Software agents are beginning to shop and pay on a person's behalf. Agent payment protocols such as AP2 produce cryptographically valid signatures for completed purchases, yet do not constrain the decisions that lead to them. Consequently, ordinary product-description text can steer a shopping agent into forming a cart that passes every protocol check but no longer matches the user's request. In this paper, we show that this vulnerability enables three related attacks. In the first attack, the agent is steered into fetching another user's payment credentials. In the second, it assembles a cryptographically valid cart whose contents do not match what the user was shown. In the third, a single factual claim about stock or product lineage moves the agent from the cheaper displayed item to a more expensive one, while the resulting cart remains fully consistent with the listing. In experiments using the Gemini Flash-Lite models that AP2's sample agents specify by default, the three attacks succeeded at rates of 90%, 56%, and 73.3%, respectively. The same vulnerability appears across seventeen Google models, three unrelated agent frameworks, two cross-vendor anchors, and Google's own consumer assistant. To address this attack vector, we introduce A-VIP (AP2 Verified-Intent Protection), a protocol-layer defense that treats the signed intent as a capability grant rather than judging the merchant's description. The defense binds every credential lookup to the session that requested it and every cart line to the listing seen, while flagging unauthorized spending. The first two attacks leave structural traces that these bindings block with zero false positives. The third attack leaves no trace, so A-VIP surfaces unauthorized spending for user confirmation. Finally, we release the A-VIP code, machine-checked invariants, and AP2-WhisperBench, a suite of 1,544 evaluation scenarios.
Text extracted automatically; images, tables and formatting may be missing. Original: https://arxiv.org/abs/2609.11757